The alkaline hydrochemical process for processing aluminosilicate ores into alumina is particularly applicable for silicon-poor bauxite, whereas it is of only limited interest for other aluminosilicate ores (silicon-rich bauxite) or not applicable (for clays, bauxite similar rocks or sillimanites), basically due to the high losses of useful components such as sodium and aluminum oxide with sodium hydrogen aluminosilicate (viz. Spravo{hacek over (c)}nik metallurga pocvetnym metallam. Proizvodstvo glinozema, M., Metallurgija, 1970, p 141).
In the production of alumina by an alkaline hydrochemical process, the silicon dioxide in the ore being treated is attached to the sodium hydrogen aluminosilicate (Na2AlzSi2O82H2O) which is insoluble in alkaline media. The formation of the sodium hydrogen aluminosilicate causes losses in the reusable components, that is, the sodium and aluminium oxide in stockpiled red mud, during the processing of high-grade bauxite. When processing low-grade bauxites with a high silicon dioxide content, the losses of reusable components are so high that they have to be further processed by sintering.
A known consecutive variant for processing silicon-rich bauxites is the sinter process from Bayer (see A. I. Lajner, Proizvodstvo glinozema, M., Metallurgija, 1961, S. 575). In this process, red mud with a high Al2O3 and Na2O content is sintered in a mixture of limestone and soda. The aluminate liquor resulting from the leaching of the sinter cake and separated from silicon is mixed with the aluminate liquor from the Bayer process for joint decomposition. Disadvantages of this process include high capital investment costs, high fuel consumption and considerable environmentally harmful emissions, wherein the composition of the red mud hinders the sintering of the prepared charge.
An alkaline process for the production of alumina from clay by sintering is known (ibid pp. 142-143). The essence of the process resides in sintering the charge consisting of clay, limestone and calcined soda. Solid sodium aluminate is formed at high temperature from the aluminium oxide contained in the charge, is converted into solution, and is subsequently precipitated out of this solution as aluminium hydroxide. Silicon dioxide is bound to dicalcium silicate which is insoluble in alkaline solution. This process, due to its high costs in materials, energy and fuel, as well as high capital costs and considerable environmentally harmful emissions, has lost industrial significance.
An alkaline-acidic process for the production of alumina from silicon-rich aluminium ores is also known (see RF-PS no. 2440296, cl. C01F7/20, published 2012). In this process, the starting material is leached out to form alkali metal hydrogen aluminosilicates that are decomposed at low temperature by treating the mud with a weak solution of a strong acid, where the aluminium and the alkali metals go into solution. Aluminium hydroxide is then separated from this solution. The principal disadvantage of the alkaline-acidic process is in the complicated recovery of the strong acids.